Conventional airplane actuator control uses Centralized Control method. On the average, a typical flight control actuator requires a wiring bundle containing 17 to 26 wires. The wires running from the flight control actuator average 100 to 125 feet long and weigh about 600 pounds. For typical engine and propulsion control, the electronics are usually contained in centralized Actuator Control Electronics (ACE) units installed in the pressurized air-conditioned fuselage. A typical wire run from those actuators to the ACE is about 15 to 20 feet. Those wire runs are difficult to design, fabricate, install and maintain. They also require a large number of connectors installed at both ends of each airframe production break. Service studies of commercial aircraft have shown that wiring and connector problems are the most frequent cause of system maintenance action in regular service.
A different control method with the potential of revolutionizing the way airplane system design is to use Distributed Control. Distributed systems move the processing and signal conditioning functions away from the central processing unit to a number of remote terminals at the engine or sensors. This method puts electronics close to control functions where cooling is usually not available. Therefore the Distributed Control requires high temperature electronics design. As an example, the proposed U.S. High Speed Civil Transport (HSCT) requires distributed actuator controllers to operate at highest ambient temperature of 193.degree. C. Distributed systems reduce the number of interconnects between central processors and sensors or effectors. In other words, use of high temperature electronics will reduce the weight of the long wires and their connectors.
There are other benefits using high temperature electronics for aerospace applications. Present electronics systems in commercial airplanes are located in environmentally controlled and centralized Electronics Equipment (EE) Bays and pressurized mid-bay fuselage. An average EE Bay cooling system may weigh 400 pound and consumes 6 KVA electrical power. It is highly desirable to eliminate, reduce or simplify the cooling requirement of the electronics boxes in the aircraft EE Bay if electronics circuit can operate with minimum active cooling, or even without such cooling at all.
In addition to aerospace applications, there are other potential commercial applications in the automobile and petroleum industries where high temperature electronics are required for engine monitoring and control, emission and exhaust control as well as data logging from geothermal instrument. High-density commercial power suppliers will also be in demand since they occupy small space and will not require heat sinking.
However, the design of high temperature electronics encounters many technical difficulties. Most silicon semiconductor integrated circuits and discrete devices are subject to their maximum junction temperature limit of 150.degree. C. Operating beyond this junction limit will cause a substantial amount of leakage in the junctions and substrate.
As operating temperature increases, circuit performance degrades rapidly. Digital parts develop additional threshold shift and more propagation delay. At high temperatures, noise margins for logic levels are reduced and flips-flops may no longer function due to additional timing violation. Analog parts, on the other hand, suffer much worse than digital counterparts at high temperatures. Excessive leakage, shift and variations of device parameters cause precision analog circuits to malfunction with devastating results.
Thanks to the Dielectric-Isolated (DI) and Silicon-On-Insulator (SOI) IC fabrication processes, originally developed for radiation hardened IC applications. The substrate current is virtually eliminated. In addition, fully depleted SOI CMOS devices have several times lower variation of the threshold voltage with operating temperatures than bulk devices. With recent modification in processing steps, SOI has become a promising technology for high temperature IC applications. As an example, the Honeywell Company has developed a precision SOI CMOS voltage reference IC, the HTREF-05.
Due to thermal limitations of the device packages, most semiconductor devices and IC's have manufacturer-specified safe operating areas and junction temperature limits that cannot be exceeded. To maintain reliable operations at high temperatures, special processing and packaging techniques such as using refractory metals and gold metallization, widening metal traces, and applying eutectic or soldering die attachment are frequently used.
Electronics circuits will not function without a properly conditioned power. High temperature power supply therefore becomes an essential part of the system. Perhaps the most challenging task is to design a high temperature switching power supply for better efficiency than a linear regulator.
At the present time, there are a handful of discrete semiconductor devices and electronics components which could operate above the 150.degree. C. temperature limit, if special processing materials and packaging techniques are used. Although it is technically difficult, with compromised circuit performance, one could still design and package an all-discrete PWM switching power supply to operate beyond the 125.degree. C. upper limit. However, discrete PWM switching power supplies, in many applications are not desirable to meet today's size, weight, and efficiency requirements.
The current industry standard PWM power supply controllers are not capable of operating at extreme temperatures. There are several PWM controllers that have been screened and tested at factories to meet the military temperature range of -55.degree. C. to +125.degree. C. However, PWM controllers operated beyond the standard military temperature ranges have not been reported anywhere. In other words, a PWM controller that operates beyond the +125 degree C in an integrated circuit form is practically nonexistent.
The present invention is directed to provide a detailed description of a power supply PWM controller that can be fabricated using the latest SOI or DI integrated circuit technology. This invention provides a method of design of a PWM controller for operation up to +225.degree. C. temperature environment. Both analyses and hardware prototypes have proved its concept and features. The PWM controller offers several unique features that are not found in the conventional PWM controller designs. It also uses fewer external components than industry standard PWM controllers do.